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POTASSIUM NITRATE                                89

KNO, = ion

Large, colourless, rhombic prisms or dry, white crystalline powder,
soluble in 4 parts of cold water, insoluble in absolute alcohol. Crude nitre
may contain various impurities (up to 20%), namely, chlorides, chlorates,
perchlorates, sulphates, nitrites, iodates, lime, magnesia, soda, copper,
and insoluble substances (sand, earthy matter). In general re-fined nitre
is fairly pure, only containing traces of chlorides. The tests to be made
are as follows :

1.  Insoluble Substances, Metals, Alkaline Earths, etc.—5 grams
are dissolved in 50 c.c. of water, filtered from any insoluble   residue   and
the filtrate tested with ammonia, ammonium sulphide, ammonium oxalate
and sodium phosphate (see also Potassium Hydroxide).

2.  Sodium Salts.—If the nitre does not colour the flame yellow it is
free from sodium salts, while if it does give a colour these salts may be
present only in traces.    The nitre is well and repeatedly triturated with
alcohol and the liquid filtered and evaporated :   in presence of sodium
nitrate, the residue is composed almost exclusively of this salt.

3.  Chlorides, Sulphates.—The solution (i : 20) is tested with silver
nitrate and with barium chloride : with pure nitre, no turbidity should be
observed even after some hours.

4.  Chlorates, Perchlorates.—Where no chlorides are present, chlorates
or perchlorates may be detected by calcining a little of the nitre, dissolving
the residue in water acidified with nitric acid and testing with silver nitrate :
a turbidity indicates chlorate or perchlorate.    If this test gives an affirmative
result, and also if the nitre contains chlorides, the chlorates and perchlorates
may be determined as follows :

(a) 5 grams of the nitre are dissolved in water, filtered if necessary,
then slightly acidified with nitric acid and the liquid titrated with silver
nitrate and decinormal thiocyanate according to Volhard's method. The
chlorine thus found exists as chlorides.

(6) 5 grams of the nitre and 10 grams of zinc dust (free from chlorine)
are gently boiled for half an hour with 150 c.c. of i% acetic acid, the liquid
being filtered and the chlorine in the filtrate determined. This chlorine is
that of the chlorides and chlorates.

(c) In a flat platinum dish, 5 grams of the nitre are mixed to a paste
with about i c.c. of pure, saturated sodium carbonate solution by means
of a platinum wire ; the paste is then dried over a small flame and heated
gradually to redness. When cool, the mass is taken up with water, the
solution being acidified with nitric acid and the chlorine again determined :
this represents the chlorine of the chlorides, chlorates and perchlorates.

5.  Iodates.—A solution of 5 grams of the nitre is acidified with dilute
sulphuric acid and zinc turnings and a little starch paste added : presence
of iodates is indicated by a blue coloration.

6.  Nitrites.—To the i : 10 solution of the nitre are added 6 drops of
i : 50 sulphuric acid, a little starch paste and 3-4 drops of potassium iodidendichromate in the solution determined iodometrically. XV, z, p, 32j, and Hull, chim. /arm,,